Strengthened glass and methods for making using differential density

ABSTRACT

Chemically strengthened glass and a method for making utilizing differential areal density are provided. The method includes providing a substrate having a glass chemical structure. Host alkali ions are situated in the chemical structure. The substrate has a treatment-rich volume and a treatment-poor volume located as opposed to each other in the substrate. The method also includes providing an exchange medium characterized by having an areal density, associated with an ion exchange rate, of invading alkali ions having an average ionic radius that is larger than an average ionic radius of the host alkali ions. The method also includes providing a modified exchange medium characterized by having a modified areal density, associated with a modified ion exchange rate, of the invading alkali ions. The method also includes applying the exchange mediums and conducting ion exchange to produce the strengthened substrate.

PRIORITY

This application claims priority to U.S. Provisional Application No.61/710,139, entitled “Strengthened Glass and Curvature Control” byPatrick K. Kreski filed on Oct. 5, 2012, which is incorporated herein byreference in its entirety.

BACKGROUND

Chemical strengthening of glass, also called ion-exchange strengtheningor chemical tempering, is a technique to strengthen a prepared glassarticle by increasing compression within the glass itself. It generallyinvolves introducing larger alkali ions into the glass chemicalstructure, to replace smaller alkali ions present in the structure. Acommon implementation of chemical strengthening in glass occurs throughthe exchange of sodium ions, having a relatively smaller ionic radius,with potassium ions, having a relatively larger ionic radius bysubmerging a glass substrate containing sodium ions in a bath containingmolten potassium salts.

Chemical strengthening is often utilized to increase compression inorder to increase strength, abrasion resistance, and/or thermal shockresistance into a glass article. The increased compression can beintroduced to various depths in the glass and is often implementedwithin a surface layer. Chemical strengthening is commonly utilized fortreating flat glass. But it may also be used for treating non-flat glassarticles, such as cylinders and other shapes of greater geometriccomplexity.

Flat glass is commonly manufactured by a number of known techniques.These include the float glass method and drawing methods, such as thefusion down-draw method and the slot draw method. However, a preparedflat glass article may have variations in its chemical compositionand/or structure at different locations in the glass. For example, flatglass that is manufactured by the float glass technique is oftenprepared by spreading softened glass material on a molten metal surfacesuch as tin. The glass is then cooled to form a solid, flat glass. As aresult, the prepared flat glass often contains a greater amount of tinon the side that was nearer the molten tin and the concentration of tinis commonly greater near the surface of that side.

Chemical strengthening is often used to treat glass having variations inchemical composition and/or structure at different locations in theglass. The variations produce locations that are treatment-rich ortreatment-poor relative to each other for ion exchange and/orcompression development in chemical strengthening. When chemicalstrengthening is used to treat such glass, the introduced compressivestress is often not uniformly distributed. This may introduce a bendingmoment and subsequent induced curvature in a glass article treated bychemical strengthening, particularly for glass articles having a widthof less than 3 mm. The induced curvature is often undesirable. Inducedcurvature is especially problematic in manufacturing thin flat glassarticles according to manufacturing specifications that include theenhanced physical properties associated with chemical strengthening, butwithout induced curvature. For example, glass used in manufacturedelectronic articles, such as displays for “smart” phones, often requiresglass that is uniformly flat and high in strength and in abrasionresistance.

For a thin, flat glass article, such as an article having two majorsurfaces, the non-equivalence of interdiffusion of invading alkali ionsand/or compression generation properties between the major surfaces ofthe flat glass substrate after chemical strengthening commonly often hasan effect, such that a local force times the distance from the mid-planeof a glass article is not equivalent when summed from the treatment-poorsurface to the mid-plane and from the rich surface to the mid-plane.Thus the net bending moment about the mid-plane is non-zero (i.e., thereis a non-zero net bending moment of the stress about the mid-plane). Asa result, bending stresses are generated. For glass articles of thincross-section, these bending stresses generate deflection of the glassarticle from flat. That is, thin, chemically strengthened glassesmanufactured by the float process often exhibit measurable curvatureafter chemical strengthening. The direction of curvature is oftenconcave on the poor surface and convex on the rich surface.

In recent years, various types of efforts have attempted to overcome theproblem of induced curvature that is associated with the chemicalstrengthening of glass. One approach involves grinding and polishing aprepared glass prior to chemical strengthening. The grinding andpolishing is performed to remove those parts of a glass having adifferent chemical composition and/or structure. An example of thisapproach is grinding and polishing a flat glass made by the float methodto remove the surface layer(s) containing a significant amount of tin.However, grinding and polishing the float glass introduces abrasions andmay introduce other physical defects, in addition to the added time andexpense associated with performing the grinding and polishing. Otherapproaches have involved secondary chemical treatments of prepared glassdone prior to chemical strengthening. The secondary chemical treatmentsare utilized in an attempt to address differences chemical compositionand/or structure at different locations in the glass. However secondarychemical treatments can alter the physical properties of the glass andotherwise degrade a glass produced through subsequent chemicalstrengthening. Also, like grinding and polishing, secondary chemicaltreatments involve the time and expense of an extra processing step thatis done prior to chemical strengthening.

Given the foregoing, chemically strengthened glass and methods formaking chemically strengthened glass are desired in which thestrengthened glass has reduced induced curvature. It is also desiredthat the strengthened glass not have the drawbacks associated withgrinding and polishing or secondary chemical treatment(s) applied inprior methods associated with the chemical strengthening of the glass.It is also desired that the strengthened glass have the improvedphysical properties of chemically strengthened glass, such as higherstrength, higher abrasion resistance, and/or higher thermal shockresistance.

SUMMARY

This summary is provided to introduce a selection of concepts. Theseconcepts are further described below in the detailed description inconjunction with the accompanying drawings. This summary is not intendedto identify key features or essential features of the claimed subjectmatter, nor is this summary intended as an aid in determining the scopeof the claimed subject matter.

According to an implementation, there is a method for making astrengthened substrate. The method may include providing a substrate.The substrate may be characterized by having a glass chemical structure.The glass chemical structure may include host alkali ions having anaverage ionic radius situated in the glass chemical structure. Thesubstrate may have dimensional volumes including a treatment-rich volumeand a treatment-poor volume. The volumes may be located as opposed toeach other in the substrate. The method may also include providing anexchange medium. The exchange medium may be characterized by having anareal density of invading alkali ions having an average ionic radiusthat is larger than the average ionic radius of the host alkali ions.The areal density may be associated with an ion exchange rate of theinvading alkali ions. The method may also include providing a modifiedexchange medium characterized by having a modified areal density of theinvading alkali ions. The modified areal density may be associated witha modified ion exchange rate that is slower than the ion exchange rate.The method may include applying the modified exchange medium to asurface of the treatment-rich volume. The method may also includeapplying the exchange medium to a surface of the treatment-poor volume.The method may also include conducting ion exchange while applying oneor more of the exchange medium and the modified exchange medium toproduce the strengthened substrate.

According to another implementation, there is an article of manufacture.The article may include a chemically strengthened substratecharacterized by having a glass chemical structure including alkali ionssituated in the glass chemical structure. The substrate may havedimensional volumes including a treatment-rich volume including a richsurface of the substrate. The volumes may also include a treatment-poorvolume including a poor surface of the substrate and characterized byhaving a variation from the treatment-rich volume in at least one of achemical composition and a chemical structure. The volumes may alsoinclude a bulk volume, within the substrate, that may be adjacent atleast one of the treatment-rich volume and the treatment-poor volume. Aconcentration of metal may be in at least one of the treatment-poorvolume and the treatment-rich volume. The concentration of metal may be≧ about 0.4 mole % higher than a concentration of the metal in the bulkvolume. A concentration of the metal may be higher in the treatment-poorvolume than a concentration of the metal in the treatment-rich volume. Aconcentration of alkali ions may be in a diffusion depth of one or moreof the treatment-rich volume and the treatment-poor volume. Theconcentration of alkali ions may be ≦about 0.5 mole % higher than aconcentration of the alkali ions in the bulk volume.

According to another implementation, there is an article of manufacture.The article may include a chemically strengthened substrate. Thechemically strengthened substrate may be made by a process includingproviding a substrate. The substrate may be characterized by having aglass chemical structure comprising host alkali ions having an averageionic radius situated in the glass chemical structure. The substrate mayhave dimensional volumes including a treatment-rich volume and atreatment-poor volume. The volumes may be located as opposed to eachother in the substrate. The process may also include providing anexchange medium. The exchange medium may be characterized by having anareal density of invading alkali ions having an average ionic radiusthat is larger than the average ionic radius of the host alkali ions.The areal density may be associated with an ion exchange rate of theinvading alkali ions. The process may also include providing a modifiedexchange medium. The modified exchange medium may be characterized byhaving a modified areal density of the invading alkali ions. Themodified areal density may be associated with a modified ion exchangerate that is slower than the ion exchange rate. The process may alsoinclude applying the modified exchange medium to a surface of thetreatment-rich volume. The process may also include applying theexchange medium to a surface of the treatment-poor volume. The processmay also include conducting ion exchange while applying one or more ofthe exchange medium and the modified exchange medium to produce thestrengthened substrate.

The above summary is not intended to describe each embodiment or everyimplementation. Further features, their nature and various advantagesare described in the accompanying drawings and the following detaileddescription of the examples and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments describedherein and, together with the description, explain these embodiments. Inaddition, it should be understood that the drawings are presented forexample purposes only. In the drawings:

FIG. 1 is a flowchart illustrating an exemplary overview of animplementation described herein;

FIG. 2 is a graph about properties of exemplary strengthened substratesmade utilizing exchange mediums applied to soda-lime silicate glass;

FIG. 3 is a graph about properties of exemplary strengthened substratesmade utilizing exchange mediums applied to sodium aluminosilicate glass;and

FIG. 4 is a flowchart illustrating exemplary processes for making astrengthened substrate.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings.The same reference numbers in different drawings may identify the sameor similar elements.

Overview

The present invention is useful for making chemically strengthenedglass, and has been found to be particularly advantageous for makingchemically strengthened glass having reduced induced curvature. Achemically strengthened glass, according to the principles of theinvention, does not have the drawbacks associated with grinding andpolishing or secondary chemical treatment(s) when done prior to chemicalstrengthening. While the present invention is not necessarily limited tosuch applications, various aspects of the invention are appreciatedthrough a discussion of various examples using this context.

FIG. 1 is flowchart illustrating an exemplary overview of animplementation described herein. Assume that a glass substrate hasvariations in its chemical composition and/or chemical structure atdifferent locations or “volumes” in the glass. One type of variation hasa chemical composition and/or chemical structure that is more readilytreated by chemical strengthening and is a “treatment-rich” volume.Another type of variation has a chemical composition and/or chemicalstructure that is less readily treated by chemical strengthening and isa “treatment-poor” volume. The term “treatment-rich volume” refers to avolume of a glass substrate which exhibits faster alkali ioninterdiffusion and/or greater compression development during chemicalstrengthening relative to a “treatment-poor volume” under equivalentchemical strengthening conditions applied to the glass substrate. Avolume may occur at a surface of a substrate, or in a space or layerbeneath the surface. A treatment-rich volume or treatment-poor volumemay be a surface layer of a glass substrate in which the diffusion ofinvading alkali ions extends to a given “diffusion depth” from thesurface, also called a penetration depth or a diffusion layer. Inchemical strengthening, a portion of the diffusion depth is incompressive stress, called case depth. Case depth is the width of thediffusion layer that is in compressive stress in a specimen.

Different exchange mediums may be utilized in performing the chemicalstrengthening of the treatment-rich volume and the treatment-poorvolume. Assume also that the different exchange mediums may bedistinguished by having different areal densities of invading alkaliions within the different exchange mediums. “Areal density”, also knownas area density or surface density is a measure of mass or moles perunit of area. An “areal density of invading alkali ions” of an exchangemedium is a measure of the moles of invading alkali ions per measure ofsurface area on a substrate to which the exchange medium is applied,such as a surface of a treatment-rich volume or treatment-poor volume ata surface of a glass substrate.

As shown in FIG. 1, at step 102, a glass substrate is provided withdifferent volumes, a treatment-rich volume and a treatment-poor volume.At step 104, an exchange medium, having an areal density of invadingalkali ions, is applied to a surface of a treatment-poor volume. At step106, a modified exchange medium having a modified areal density, onethat is modified to be a lesser areal density of invading alkali ionsthan the exchange medium applied in step 104, is applied to a surface ofa treatment-rich volume in the glass substrate. An areal density ofinvading alkali ions in an exchange medium may be modified by changingat least one of (1) the volume of the exchange medium applied to theseparate surfaces or (2) the concentration of invading alkali ions inthe exchange mediums applied to the separate surfaces of the substrate.

While the exchange mediums are applied, chemical strengthening proceedsto produce a strengthened substrate in which the induced curvature hasbeen reduced or nullified through the application of the differentexchange mediums to the different volumes. Without wishing to be boundby any particular theory, it appears that the lower areal density ofinvading alkali ions applied to the treatment-rich volume offsets thedifference in ion-exchangeability between the treatment-rich andtreatment-poor volumes, thus reducing or nullifying induced curvaturethat may otherwise result from chemical strengthening of the glasssubstrate.

For simplicity and illustrative purposes, the present invention isdescribed by referring mainly to embodiments, principles and examplesthereof. In the following description, numerous specific details are setforth in order to provide a thorough understanding of the examples. Itis readily apparent however, that the embodiments may be practicedwithout limitation to these specific details. In other instances, someembodiments have not been described in detail so as not to unnecessarilyobscure the description. Furthermore, different embodiments aredescribed below. The embodiments may be used or performed together indifferent combinations.

The operation and effects of certain embodiments can be more fullyappreciated from the examples, as described below. The embodiments onwhich these examples are based are representative only. The selection ofthese embodiments to illustrate the principles of the invention does notindicate that materials, components, reactants, conditions, techniques,configurations and designs, etc. which are not described in the examplesare not suitable for use, or that subject matter not described in theexamples is excluded from the scope of the appended claims or theirequivalents. The significance of the examples may be better understoodby comparing the results obtained therefrom with potential results whichmay be obtained from tests or trials that may be, or may have been,designed to serve as controlled experiments and to provide a basis forcomparison.

As used herein, the terms “based on”, “comprises”, “comprising”,“includes”, “including”, “has”, “having” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B is true (orpresent). Also, use of the “a” or “an” is employed to describe elementsand components. This is done merely for convenience and to give ageneral sense of the description. This description should be read toinclude one or at least one and the singular also includes the pluralunless it is obvious that it is meant otherwise.

The meaning of abbreviations and certain terms used herein is asfollows: “mm” means millimeter(s), “μm” means micrometer(s) ormicron(s), “g” means gram(s), “mg” means milligram(s), “μg” meansmicrogram(s), “L” means liter(s), “mL” means milliliter(s), “cc” meanscubic centimeter(s), “cc/g” means cubic centimeters per gram, “mol”means mole(s), “mmol” means millimole(s), “wt %” means percent by weightand “mol %” means percent by mole.

Exemplary Substrate Glasses

As used herein a “glass substrate” may comprise any kind ofion-exchangeable glass. Examples of such glass include soda-limesilicate glass, alkali aluminosilicate glass or alkalialuminoborosilicate glass, though other glass compositions arecontemplated including glasses where glass forming components are freeof silica, such as boron oxide (borate), phosphorous oxide (phosphate),aluminum oxide (aluminate), etc. As used herein, “ion exchangeable”means that a glass substrate is capable of exchanging alkali ion locatedin the glass structure of the substrate (i.e., “host alkali ions”), suchas at or near the surface of the substrate, with larger alkali ions(i.e., “invading alkali ions”) from an exchange medium that may be aliquid, solid or gas. An “ion exchange rate” refers to an amount ofinvading ions entering a substrate over a period of time. A glass mayhave chemical composition and/or chemical structure variations atdifferent locations or “volumes” in the glass. An example of chemicalcomposition variation is an excess of metal, such as metal ions or otherforms of metal and may include a metal species, such as tin or lead. Anexample is metal that remains in a flat glass made by a float glassmethod, such as tin. An example of chemical structure variation is thepresence of an element in the glass in which the element may havedifferent valences throughout different volumes, such as tin present inSn²⁺ and Sn⁴⁺ valences in the different volumes. In this example, thedifferent forms of tin form different chemical structures in thedifferent volumes.

Exemplary embodiments of substrate glasses include silicate glasses,such as soda-lime silicate glass or sodium aluminosilicate glass thatincludes alumina, at least one alkali metal and, in some embodiments,greater than 50 mol % SiO₂, in other embodiments at least 58 mol % SiO₂,and in still other embodiments at least 60 mol % SiO₂.

Exemplary Strengthened Glasses

Exemplary embodiments of chemically strengthened glasses includesoda-lime silicate glass and sodium aluminosilicate glass which arestrengthened, such as, in potassium nitrate salt baths. Chemicalstrengthening may be performed at various temperatures, such as attemperatures above about 400° C., preferably about 430° C., and with ionexchange durations of about 1-24 hours. The zone of compressive stressoccurs, for example, within a diffusion depth of about 5, 10, 20, 30,40, 50, 60, 70, 80, 90, 100 or 125 to about 150 μm of a surface of asubstrate glass. According to an exemplary embodiment, compressivestress in a strengthened glass is greatest at a surface (i.e., a“surface compression”) of the glass and the level of compressive stressfollows a gradient extending downward from the surface through a casedepth in the strengthened glass. In exemplary embodiments, the amount ofsurface compression may be up to about 800 MPa or higher in strengthenedsoda-lime silicate glass and up to about 1200 MPa or higher inaluminosilicate glass. In some exemplary embodiments, surfacecompression is about 200-650 MPa in strengthened soda-lime silicateglass and about 300-850 MPa in aluminosilicate glass. In other exemplaryembodiments, surface compression is about 400-600 MPa in strengthenedsoda-lime silicate glass and about 600-800 MPa in aluminosilicate glass.

In some exemplary embodiments, a strengthened silicate glass, such assoda-lime silicate glass or sodium aluminosilicate glass comprisesalumina, at least one alkali metal and, in some embodiments, greaterthan 50 mol % SiO₂, in other embodiments at least 58 mol % SiO₂, and instill other embodiments at least 60 mol % SiO₂. In these embodiments, aLi₂O+Na₂O+K₂O total mol %, such as in a volume associated with adiffusion depth, is at least about 1, 2, 5, 7 or 8-10 mol % and ≦25 mol%, preferably ≦20 mol %, and more preferably ≦about 2, 5, 7, 8, 10, 12,or 16-18 mol %.

In another exemplary embodiment, an alkali aluminosilicate glasscomprises, consists essentially of, or consists of: 60-75 mol % SiO₂;5-15 mol % Al₂O₃; 0-12 mol % B₂O₃; 8-21 mol % Na₂O; 0-8 mol % K₂O; 0-15mol % MgO; and 0-3 mol % CaO. In these embodiments, such as in a volumeassociated with a diffusion depth, a Li₂O+Na₂O+K₂O total mol % is atleast about 1, 2, 5, 7 or 8-10 mol % and ≦25 mol %, preferably ≦20 mol%, and more preferably ≦about 2, 5, 7, 8, 10, 12, 15 or 16-18 mol %.

In yet another embodiment, an alkali aluminosilicate glass substratecomprises, consists essentially of, or consists of: 60-70 mol % SiO₂;6-14 mol % Al₂O₃; 0-15 mol % B₂O₃; 0-15 mol % Li₂O; 0-20 mol % Na₂O;0-10 mol % K₂O; 0-15 mol % MgO; 0-10 mol % CaO; 0-5 mol % ZrO₂; 0-2 mol% SnO₂; 0-1 mol % CeO₂; wherein about 1, 2, 5, 7, 8, or 10-12 mol%≦Li₂O+Na₂O+K₂₀≦about 2, 5, 7, 8, 10, 12, 15 or 16-20 mol %, such as ina volume associated with a diffusion depth, and 0 mol %≦MgO+CaO≦15 mol%.

In one example embodiment, sodium ions in the substrate glass arereplaced by potassium ions from a molten bath, though other alkali metalions having a larger atomic radius, such as rubidium or cesium, mayreplace smaller alkali metal ions in the glass. Similarly, other alkalimetal salts such as, but not limited to, nitrates, sulfates, halides,and the like may be used in the ion exchange process.

In another example embodiment, a chemically-strengthened glass substratecan have a surface compressive stress of about 200 MPa or more, e.g.,about 300, 400, 500, 600, 700, 800, 900, 1000 or 1500 MPa or more, acase depth of about 5 μm or more (e.g., about 5, 10, 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 μm or more) and adiffusion depth of about 5 μm or more (e.g., about 5, 10, 20, 25, 30,35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 125 or 150 μmor more).

In another example embodiment, a chemically-strengthened glass substratecan have a higher amount of metal in at least one surface volume orlayer, such as a treatment-rich volume or a treatment-poor volume, thanin a bulk volume adjacent these surface volumes. A concentration ofmetal in at least one of the treatment-poor volume and thetreatment-rich volume may be ≧ about 0.4, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0,5.0, 6.0, 8.0, 10.0, 12.0, 15.0, 20 or 25 mol % higher than aconcentration of the metal in the bulk volume. According to anembodiment, a concentration of metal in the treatment-poor volume ishigher than a concentration of the metal in a treatment-rich volume. Acommon example of strengthened glass with variant metal concentrationsin the different volumes is chemically strengthened glass made from aflat glass substrate prepared using a float glass process utilizing tin.

In another example embodiment, a chemically-strengthened glass substratemay have an average concentration of alkali ions (e.g. invading alkaliions and host alkali ions) that is the same or different in a diffusiondepth of a surface volume than in an adjacent volume, such as a bulkvolume. The surface volume may be a treatment rich volume or atreatment-poor volume in the strengthened glass. The averageconcentration of alkali ions may be the same or different from anaverage concentration of alkali ions in the adjacent volume, such as abulk volume. In one example embodiment, the average concentration ofalkali ions in the diffusion depth of the surface volume is ≦to about0.5 mol % higher than a concentration of the alkali ions in the bulkvolume. In other embodiments, the average concentration of alkali ionsin the diffusion depth of the surface volume is ≦to about 0.4, 0.3, 0.2,0.1 or 0.05 mol % higher, equal to or less than a concentration of thealkali ions in the bulk volume adjacent the surface volume.

Exemplary Exchange Mediums

Exemplary embodiments of a liquid exchange medium which may be utilizedin chemical strengthening include liquid molten salt baths. The moltenliquid baths include invading alkali ions having an average ionic radiusin the alkali metal ion of the molten salt that is larger than anaverage ionic radius of host alkali metal ions in the substrate glassprior to ion exchange. A common example of a liquid molten salt bathincludes potassium nitrate with potassium as the invading alkali ion toreplace sodium and/or lithium host ions in the substrate glass.

Mixed salt blends of invading alkali ions may also be used as liquidexchange mediums. These blends may include salts of different alkalimetals, preferably different alkali metal nitrates. A nitrate melt blendmay include at least two different alkali ions, for example Na and K, oras well Na and Rb. But it is also possible that three or four differentalkali metals are included. Rb ions or Cs ions may be used in chemicalstrengthening. The method according to the embodiment offers the optionto effectively incorporate invading alkali ions into a treated glassarticle having ionic radii that are significantly larger than the radiiof host alkali ions, such as lithium or sodium ions.

Exemplary embodiments of a solid exchange medium which may be utilizedin chemical strengthening include semi-solid pastes that may be appliedto a surface of a glass substrate. The paste includes invading alkaliions from a source such as a salt and at least one rheological agent,such as clay, to suspend the ions in the solid exchange medium. Kaolinis a common example of a rheological agent which may utilized in makinga solid exchange medium. The viscosity of a paste made with kaolin maybe modified with water and other additives to suit an application bywhich the paste is applied to a glass substrate. Water content of apaste may be evaporated prior to application as a solid exchange mediumutilizing a raised high temperature, such as greater than 120° C.Another example of a rheological agent is aluminosilicate fiber. Otherclays and rheological agents are also contemplated.

In addition to liquid and solid exchange mediums, gas exchange mediumsare also contemplated.

EXAMPLES

The following examples demonstrate methods of making chemicallystrengthened glass utilizing differential areal density methodology.

Example 1

Example 1 demonstrates the preparation of a chemically strengthenedsoda-lime silicate glass having a reduced induced curvature. Referenceis made to graph 200 in FIG. 2 in the example. Graph 200 shows resultsassociated with a differential areal density methodology. An arealdensity of KNO₃ in a paste applied to a “rich surface” (i.e., a surfaceof a treatment-rich volume in a substrate) is given in grams/2,500 mm²on the abscissa. Deflection for a flat glass having 50 mm span is givenin microns on the ordinate. Note that for this example, an idealflatness, (i.e., a near zero induced curvature) is crossed between 0.030and 0.040 grams/2,500 mm², as shown by the data plotted in graph 200.

Sample preparation: Soda-lime silicate glass coupons, 50 mm×50 mm acrossand 0.4 mm width, were cut from a mother sheet formed by a tin floatglass process. Both surfaces (i.e., the rich surface and the poorsurface) of the coupons were manually coated with a uniformly levellayer of the KNO₃-containing paste as a solid exchange medium, howeverdifferent volumes of the paste were applied to the different sides ofdifferent coupons to demonstrate the effect of varying the areal densityby varying the volume of the paste applied. The differences in volumewere measured in terms of mass applied on each side.

The areal density of KNO₃ in paste applied on the poor surface was heldconstant at 2,520 g/2500 mm² and the areal density of KNO₃ in pasteapplied on the rich surface varied on separate coupons: 0.020, 0.030,0.040, 0.063, 0.125, and 0.250 g/2500 mm². After drying the pastes onthe coupons, all coupons were processed in air at 440° C. for 24 hours.After ion exchange treatment, the treated coupons were removed,air-cooled, and rinsed with water to remove the paste. A minimum of twocoupons were examined for each parameter.

Results: Coupon deflection after processing was determined from surfaceprofiles measured using a non-contact optical profiler. Deflection isthe peak-to-valley height determined along a line drawn between oppositeedge mid-points of the square coupon. Deflection versus areal density ofKNO₃ in paste is given in graph 200. A positive deflection measurementin graph 200 indicates convex curvature of the rich surface. A negativedeflection measurement indicates concave curvature of the rich surface.

For commercial purposes, such as for utilization in personal electronicdevices and flat panel displays, an acceptable deflection is about 0.1%of the linear span—corresponding to 50 microns for a 50 mm span. TheKNO₃ areal density of KNO₃ in the paste of 0.020, 0.030, 0.040, and0.063 g/2500 mm² produced a deflection measurement of less than the 50micron target in this example. The ideal flat was crossed between 0.030and 0.040 g/2500 mm². At 0.030 g/2500 mm², average deflection was −22.6micron (rich side concave), average surface compression was 333 MPa forthe rich surface and 347 MPa for the poor surface, average case depthwas 24.3 micron for the rich surface and 23.5 micron for the poorsurface.

Comparative sample coupons, which had 2.500 g/2500 mm² KNO₃ in the pasteapplied to the rich surface and 2.500 g/2500 mm² KNO₃ in the pasteapplied to the poor surface, and otherwise underwent equivalentprocessing had average deflection of 196 micron (rich side convex),average surface compression was 412 MPa for the rich surface and 379 MPafor the poor surface, and average case depth was 27.1 micron on the richsurface and 27.9 micron on the poor surface. The average deflection ofthe comparative sample coupons is represented by the horizontal dashedline in graph 200 of FIG. 2.

Example 2

Example 2 demonstrates the preparation of a chemically strengthenedaluminosilicate glass having a reduced induced curvature. Reference ismade to graph 300 in FIG. 3. Graph 300 shows results associated with adifferential areal density methodology. An areal density of KNO₃ in apaste applied to a “rich surface” (i.e., a surface of a treatment-richvolume in a substrate) is given in grams/2,500 mm² on the abscissa.Deflection for a flat glass having 50 mm span is given in microns on theordinate. Note that for this example, an ideal flatness, (i.e., a nearzero induced curvature) is crossed between 0.675 and 0.765 grams/2,500mm², as shown by the data plotted in graph 300.

Sample Preparation: Sodium aluminosilicate glass coupons, 50 mm×50mm×0.56 mm width, were cut from a mother sheet formed by the tin floatprocess. Both surfaces (i.e., the treatment rich surface and thetreatment-poor surface) of the coupons were manually coated with auniformly level layer of the KNO₃-containing paste as a solid exchangemedium. However different volumes of the paste were applied to thedifferent sides to demonstrate the effect of varying the areal densityby varying the volume of the paste applied. The differences in volumewere measured in terms of mass applied on each side.

The areal density of KNO₃ in paste applied on the poor surface was heldconstant at 2.520 g/2500 mm² and the areal density of KNO₃ in pasteapplied on the rich surface varied on separate coupons: 0.375, 0.450,0.525, 0.600, 0.675, and 0.765 g/2500 mm². After drying the pastes onthe coupons, all coupons were processed in air at 450° C. for 2 hours.After the exchange treatment, the coupons were removed, air-cooled, andrinsed with water to remove the paste. A minimum of two coupons wereexamined for each parameter.

Results: Coupon deflection after processing was determined from surfaceprofiles measured using a non-contact optical profiler. Deflection isthe peak-to-valley height determined along a line drawn between oppositeedge mid-points of the square coupon. Deflection versus pre-treatmenttime is given in graph 300. A positive deflection measurement in graph300 indicates convex curvature of the rich surface. A negativedeflection measurement indicates concave curvature of the rich surface.

For commercial purposes, such as for utilization in personal electronicdevices and flat panel displays, an acceptable deflection is about 0.1%of the linear span—corresponding to 50 microns for a 50 mm span. TheKNO₃ areal density of 0.450, 0.525, 0.600, 0.675, and 0.765 g/2500 mm²produced a deflection measurement of less than the 50 micron target inthis example. The ideal flat was crossed between 0.675 and 0.765 g/2500mm². At 0.675 g/2500 mm², average deflection was −20.6 micron (rich sideconcave), average surface compression was 704 MPa for the rich surfaceand 700 MPa for the poor surface, average case depth was 37.9 micron forthe rich surface and 37.8 micron for the poor surface.

Comparative sample coupons, which had 2.519 g/2500 mm² KNO₃ in pasteapplied to the rich surface and 2.519 g/2500 mm² KNO₃ in paste appliedto the poor surface, and otherwise underwent equivalent processing hadaverage deflection of 41.4 micron (rich side convex), average surfacecompression was 737 MPa for the rich surface and 743 MPa for the poorsurface, and average case depth was 35.3 micron on the rich surface and36.2 micron on the poor surface. The average deflection of thecomparative sample coupons is represented by the horizontal dashed linein graph 300 of FIG. 3.

FIG. 4 is flowchart illustrating exemplary processes for making astrengthened substrate.

At step 402, a glass substrate is provided having different volumes,such as a “treatment-rich” volume and a “treatment-poor” volume in theglass structure including host alkali ions. The glass may be soda-limesilicate glass or aluminosilicate glass. The volumes may be located, forexample, as opposed to each other in the substrate, and according to anembodiment, may be diametrically opposed. The glass substrate may havevariations in the different volumes, such as a variation in chemicalcomposition and/or chemical structure. An example of a variation inchemical composition is an amount of tin situated in different volumesof the glass. An example of a variation in chemical structure is thepresence of tin in different valences, Sn²⁺ and Sn⁴⁺ in differentvolumes of the glass. A variation in chemical composition and/orchemical structure in the treatment-poor volume may distinguish it fromthe treatment-rich volume.

At step 404, a solid exchange medium having an areal density (shown as“AD-1” in FIG. 4) of invading alkali ions is applied to a surface of thetreatment-poor volume. In another embodiment, a liquid exchange mediumhaving areal density AD-1 of invading alkali ions is applied to asurface of the treatment-poor volume.

At step 406, according to an embodiment, a solid exchange medium havinga modified areal density (shown as “AD-2” in FIG. 4) of invading alkaliions is applied to a surface of the treatment-rich volume. The modifiedexchange medium has a smaller volume than the exchange medium applied instep 404, but is otherwise equivalent to the exchange medium applied instep 404.

Step 408 demonstrates an alternative embodiment/step than step 406. Atstep 408, according to the alternative embodiment, a solid exchangemedium having a modified areal density (shown as “AD-2” in FIG. 4) ofinvading alkali ions is applied to a surface of the treatment-richvolume. The modified exchange medium has a smaller concentration ofinvading alkali ions than the exchange medium applied in step 404, butis otherwise equivalent to the exchange medium applied in step 404.

At step 410, ion exchange is conducted on the glass substrate with theexchange medium and the modified exchange medium applied to therespective surfaces. According to an embodiment, a net bending momentabout midplane in the substrate is about zero in a fully strengthenedsubstrate.

Although described specifically throughout the entirety of thedisclosure, the representative examples have utility over a wide rangeof applications, and the above discussion is not intended and should notbe construed to be limiting. The terms, descriptions and figures usedherein are set forth by way of illustration only and are not meant aslimitations. Those skilled in the art recognize that many variations arepossible within the spirit and scope of the principles of the invention.While the examples have been described with reference to the figures,those skilled in the art are able to make various modifications to thedescribed examples without departing from the scope of the followingclaims, and their equivalents.

What is claimed is:
 1. A method for making a strengthened substrate, themethod comprising: applying an exchange medium to a surface of atreatment-poor volume of a substrate, wherein the substrate has a glasschemical structure including host alkali ions having an average ionicradius situated in the glass chemical structure, wherein thetreatment-poor volume is one of dimensional volumes included in thesubstrate, and wherein the exchange medium has an areal density ofinvading alkali ions having an average ionic radius that is larger thanthe average ionic radius of the host alkali ions, and the areal densityis associated with an ion exchange rate of the invading alkali ions;applying a modified exchange medium to a surface of a treatment-richvolume of the substrate, wherein the treatment-rich volume is one of thedimensional volumes included in the substrate, and the treatment-richvolume and the treatment-poor volume are located as opposed to eachother in the substrate, wherein a modified exchange medium has amodified areal density of the invading alkali ions, and the modifiedareal density is associated with a modified ion exchange rate of theinvading alkali ions, and wherein the modified ion exchange rate isslower than the ion exchange rate; conducting first ion exchange whileapplying the exchange medium; and conducting second ion exchange whileapplying the modified exchange medium, wherein the first ion exchangeand the second ion exchange are conducted concurrently to produce thestrengthened substrate.
 2. The method of claim 1, wherein the modifiedion exchange rate is associated with a concentration of the invadingalkali ions in the modified exchange medium that is lower than aconcentration of the invading alkali ions in the exchange medium.
 3. Themethod of claim 1, wherein the modified ion exchange rate is associatedwith a volume of the modified exchange medium applied to thetreatment-rich surface that is smaller than a volume of the exchangemedium applied to the treatment-poor surface.
 4. The method of claim 1,wherein the modified ion exchange rate is associated with at least oneof a concentration of the invading alkali ions in the modified exchangemedium that is lower than a concentration of the invading alkali ions inthe exchange medium and a volume of the modified exchange medium appliedto the treatment-rich surface that is smaller than a volume of theexchange medium applied to the treatment-poor surface.
 5. The method ofclaim 1, wherein the substrate comprises a variation in at least one ofchemical composition and chemical structure in the substrate wherein atleast one of the chemical composition and chemical structure in thetreatment-poor volume is different than in the treatment-rich volume. 6.The method of claim 1, wherein a net bending moment about a mid-planedue to stress from the first and second ion exchanges is about zero inthe strengthened substrate.
 7. The method of claim 1, wherein acompressive stress varies at different locations of the strengthenedsubstrate.
 8. The method of claim 1, wherein each of the first ionexchange and the second ion exchange is performed at a constanttemperature.
 9. The method of claim 1, wherein each of the first ionexchange and the second ion exchange is performed while applying aboutan equal temperature to the treatment-poor volume and the treatment-richvolume.
 10. The method of claim 1, wherein each of the exchange mediumand the modified exchange medium is one of a liquid, a solid, a gas andmixtures thereof.
 11. The method of claim 1, wherein the treatment-richvolume and the treatment-poor volume are located as diametricallyopposed in the substrate.
 12. The method of claim 1, wherein the methodis one of a continuous process and a batch process.
 13. The method ofclaim 1, wherein the substrate comprises one of alkali aluminosilicateglass and soda-lime silicate glass.
 14. The method of claim 1, whereinthe substrate is flat.
 15. The method of claim 1, wherein the substratehas a width of about 3.0 millimeters or less.
 16. The method of claim 1,wherein each of the exchange medium and the modified exchange medium isapplied in a paste.